The effect of Ni:Co ratio on the elemental phase partitioning in γ-γ′ Ni-Co-Al-Ti-Cr alloys

Atom probe tomography has been used to characterise the effect of varying Ni:Co ratio on elemental phase partitioning at 800 °C in γ-γ′ alloys derived from the Ni-Co-Al-Ti-Cr system. In all alloys tested, Al and Ti were found to partition preferentially to the γ′ phase, whereas Co and Cr partitioned preferentially to the γ phase. However, above a critical Co content (∼19 at.%), the extent of partitioning of Al and Ti to the γ′ phase reduced. Conversely, Cr partitioned more strongly to the γ phase with Co additions of up to ∼19 at.%, above which this preferential segregation was less pronounced. This non-monotonic trend of elemental partitioning behaviour with increasing Co concentration was attributed to a transition in the chemistry of the L12 γ′ phase from Ni3(Ti, Al) to (Ni, Co)3(Ti, Al) and thus to a change in its solute solubility.